In the electrolytic cells of the diaphragm type, such as the type of cell described in U.S. Pat. No. 1,866,065, the anode compartment is separated from the cathode compartment by a permeable diaphragm. Alkali metal chloride brine, such as lithium, sodium, or potassium chloride, is introduced into the anode compartment, where it comes into contact with the anodes, and is caused to percolate through the diaphragm into the cathode compartment, where it comes into contact with the cathodes. When an electric current is passed between these electrodes, chlorine is liberated at the anodes and alkali metal hydroxide is formed at the cathodes with the liberation of hydrogen. In order to minimize voltage drop in the cell, the cathodes are placed as close to the diaphragm as possible; and in fact, in practice the diaphragm is generally a thin sheet of fibrous material, preferably of asbestos, overlying and supported by cathodes of woven iron wire screen. The cathode compartment may be occupied by hydrogen; but in best modern practice it is allowed to fill up with caustic alkali solution to a level at which the diaphragm is largely submerged, and to overflow from the cell at that level. In any case, the surface of the cathodes in contact with the diaphragm is wet with catholyte.
A review of the electrochemistry of deposited diaphragm type cells has been given by Murray, R. L. and Kircher, M. S. in the Transactions of the Electrochemical Society; vol. 86, pp. 83-106; 1944.
Chlorine, as such and as hypochlorous acid, is more or less soluble in brine, even at elevated temperatures, and forms hypochlorites n accordance with the following representative equations EQU H.sub.2 O + Cl.sub.2 .fwdarw. H.sup.+ + Cl.sup.- + HClO (1) EQU hclO .fwdarw. H.sup.+ + ClO.sup.- (2) EQU clO.sup.- + Na.sup.+ .fwdarw. NaClO (3)
thus, some chlorine inevitably passes through the diaphragm in solution in the percolating brine. When coming into contact with the caustic alkali in the cathode compartment, this chlorine reacts with the alkali to form alkali metal hypochlorite, in accordance with the following representative equations EQU Na.sup.+ + 2OH.sup.31 + Cl.sub.2 .fwdarw. NaClO + Cl.sup.- + H.sub.2 O (4) EQU na.sup.+ + OH.sup.- + HClO .fwdarw. NaClO + H.sub.2 O (5)
and, more likely, the hypochlorite is completely ionized; EQU Na.sup.+ + OH.sup.- + HClO .fwdarw. Na.sup.+ + ClO.sup.- + H.sub.2 O
this, of course, represents a loss of both chlorine and caustic alkali as useful product; hence a loss in current efficiency.
A more serious loss comes about through back migration from the cathode compartment of the cell, or face of the cathode itself, through the diaphragm to the anode compartment, chiefly of negatively charged hydroxyl ions seeking their way to the positive anode. Hydroxyl ions reaching the anodes as such are then discharged, liberating oxygen. However, at normal anolyte pH, which is about 4, hypochlorous acid may be formed by reaction with the chlorine in accordance with the following equation EQU OH.sup.- + Cl.sub.2 .fwdarw. HClO + Cl.sup.- ( 6)
Furthermore, since the hydroxyl ions carry a negative charge, which is discharged at the anode, their back migration represents a loss in current efficiency.
Hypochlorite ion, which is formed from the hydrolysis of chlorine dissolved in the anolyte, is discharged at the anode to form chlorate ion in a manner after the following equation EQU 12ClO.sup.- + 6H.sub.2 O .fwdarw. 4ClO.sub.3.sup.- + 8Cl.sup.- + 12H.sup.+ + 3O.sub.2 + 12e (7)
Further, hypochlorous acid and hypochlorite ion are unstable under the conditions of electrolysis and tend to form chlorate ion and oxygen according to the following equations EQU ClO.sup.- + 2HClO .fwdarw. ClO.sub.3.sup.- + 2Cl.sup.- + 2H.sup.+ (8) EQU 2hclO .fwdarw. O.sub.2 + 2Cl.sup.- + 2H.sup.+ ( 9)
the oxygen produced from hydroxyl ions discharging at the anode and from the decomposition of some of the hypochlorites in the anolyte thereby results in contamination of the chlorine; also, since the anodes are of graphite, some of the oxygen attacks the anodes, slowly consuming them, which results in the contamination of the chlorine with carbon dioxide. Similarly, the oxygen produced from the decomposition of some of the hypochlorites in the catholyte results in contamination of the hydrogen with oxygen.
In the cathode compartment substantial amounts of the hypochlorite and chlorate ions are reduced by nascent hydrogen (H.sup.0) formed at the cathode according to the following equations EQU 2H.sup.0 + ClO.sup.- .fwdarw. Cl.sup.- + H.sub.2 O (10) EQU 6h.sup.0 + clO.sub.3.sup.- .fwdarw. Cl.sup.- + 3H.sub.2 O (11)
however, some of the hypochlorite and chlorate ions escape reduction in the catholyte and pass out of the cell and thereby contaminate the cell effluent which is mainly spent brine having the alkali metal hydroxide dissolved therein.
In the presence of an excess of alkali, the chlorate is quite stable. It therefore tends to persist in the cell effluent and to pass on through to the evaporators in which the caustic alkali is concentrated. Practically all of the chlorate survives the evaporation and remains in the final product, where it constitutes a highly objectionable contaminant, especially to the Rayon industry.
The problem of lowering chlorates has been attacked at two main points
a. The chlorates having been formed, can be reduced in the further processing of the caustic alkali and by special treating methods. See for instance, U.S. Pat. Nos. 2,622,009; 2,044,888; 2,142,670; 2,207,595; 2,258,545; 2,403,789; 2,415,798; 2,446,868; and 2,562,169; and British Pat. Nos. 642,946 and 664,023 which show representative examples of different methods used for reducing the chlorates after they have been formed.
b. The production of chlorates during the electrolysis can be lowered by adding a reagent to the brine feed which reacts preferentially with the back migrating hydroxyl ions from the cathode compartment of the cell making their way through the diaphragm into the anode compartment, and by such a reaction prevents the formation of some of the hypochlorites in the manner shown by Equation 6 and thus additionally preventing these hypochlorites from further reacting to form chlorates in the manner shown by Equations 7, 8, and 9. Reagents such as hydrochloric acid shown in U.S. Pat. No. 583,330, and sulfur in an oxidizable form, such as sodium tetrasulfide, shown in U.S. Pat. No. 2,569,329 are illustrative of methods which have been used to attack the problem of chlorates in caustic by removing the back migrating hydroxyl ions before they can react to form chlorates.
Another method of controlling chlorate in alkali metal hydroxides in U.S. Pat. No. 2,823,177. In this patent, nickel or cobalt metal or salts thereof in finely divided form are incorporated into a cell diaphragm when it is being constructed outside of the cell in which it is to be utilized. This nickel or cobalt in the diaphragm is believed to be converted to the insoluble hydroxide form in the diaphragm which then acts catalytically to reduce chlorate formation in an operating electrolytic cell by decomposing the precursor hypochlorite before it can converted to chlorate. This process disclosed in U.S. Pat. No. 2,823,177 is effective for a period of time less than the period of time the diaphragm itself is useful in the electrolysis and thus the operation of the cell must be stopped and the diaphragm replaced if low chlorate alkali metal hydroxide is to be obtained. The length of life for a diaphragm of the type disclosed in U.S. Pat. No. 2,823,177 depends on the degree of nickel loading in the diaphragm, the form of the nickel or cobalt as well as the production rate of the cell and the life can be prematurely ended by poisoning of the nickel or cobalt hydroxide catalyst during upsets to the system. In commercial operation, cells employing such nickel or cobalt containing diaphragms have been found to be fully operational from one to two months before they must be replaced with the accompanying shutdown.
The invention of the present application on the otherhand utilizes only nickel values in the brine feed periodically to continuously maintain minimal chlorate formation thus eliminating excessive chlorate formation as a life determining factor in operation of such an electrolytic cell. Likewise, the present method of minimizing chlorate production is less critical in that the nickel values are supplied more evenly to the diaphragm since the nickel is dissolved in the brine feed whereas the closest prior art patent attempts to obtain uniformity by mixing finely divided nickel solids with the material of the diaphragm during construction thereof. The use of solid particulate nickel values by the prior art method of necessity results in the use of excess nickel as compared to the use of dissolved nickel values in accordance with the instant invention.